Conventionally, a friction material such as brake is produced by using a fiber base material, a friction modifier and a binder, blending them, and performing a production process including steps such as preforming, thermoforming and finishing. In the friction material such as brake, a fiber base material, e.g., an organic fiber such as aramid fiber, an inorganic fiber such as glass fiber, or a metal fiber such as copper fiber, an organic friction modifier such as rubber dust and cashew dust, an inorganic friction modifier such as calcium carbonate, barium sulfate, metal particle, ceramic particle and graphite, and a binder such as phenol resin, are used. Among them, graphite as a friction modifier when used as an ingredient of a friction material, not only ensures a stable friction coefficient in various environments due to its lubricity and in particular its excellent thermal stability and chemical stability but also exhibits excellent performance in terms of wear resistance and seizure resistance and enables reduction of various noises. Therefore, various kinds of graphite are blended in a friction material.
For example, Patent Document 1 describes a technique of blending, as a filler in a friction material, graphite having high elasticity such that the percentage of volume compression when applying a load of 9,000 kg/cm2 is 80% or more based on application of a load of 0.1 kg/cm2 and the percentage of recovery after removing the load is 50% or more.
Patent Document 2 describes a non-asbestos friction material using a fiber base material, a friction modifier and a binder, in which from 0.5% by volume to 2.5% by volume of a partially graphitized coke is blended.
Patent Document 3 describes a friction material containing an inorganic abrasive and a lubricant, in which an inorganic particle having a Mohs hardness of 5 to 8 (e.g., zirconium oxide) and an average particle diameter of 0.5 μm to 10 μm is contained as the inorganic abrasive in an amount of 0.5% by volume to 10% by volume based on the whole amount of the friction material, graphite and petroleum coke are contained as part of the lubricant in an amount of 8% by volume to 15% by volume in total based on the whole amount of the friction material, and the ratio of graphite and petroleum coke is from 2:8 to 3:7 by volume.
In the case of a non-asbestos friction material, a metal having large thermal conductivity, in particular a copper fiber or a copper particle, is sometimes added in order to enhance the fading resistance.
A fading phenomenon is caused by a decomposition gas generated resulting from decomposition of an organic substance contained in the friction material upon exposure of the friction material to high temperature and high load, and when copper having large thermal conductivity is added to the friction material, the heat radiation property of the friction material itself is increased, so that the generation of decomposition gas can be suppressed.
However, from concerns about environmental pollution such as river or sea pollution and adverse effects on the human body depending on the kind of metals, a friction material containing no heavy metal such as copper has been developed, and a move toward removing a heavy metal is recently accelerated on an international basis.
Patent Document 4 describes a non-asbestos friction material containing neither copper nor a copper alloy as a blending material for the raw material of the friction material and containing graphite and coke in an amount of 5% by volume to 25% by volume in total, in which the volume ratio of graphite/coke is from 2:1 to 7:1 and the average particle diameter of the coke is from 150 μm to 400 μm.